Process of coating aluminum electrolytically



sept. 1o, 1929. j C. L. BEM. 1,727,331

PROCESS QF COATING ALUMINUM ELECTROLYTICALLY Filed Jan. 1'?. 1927 .ilweaqueous aad bath Subsantzay free #om aUfaZL lsalzsuchj as L 5% sulfuric9 acid SOZzLOn/V//T/ he cathde.

ATTORNEY.

Patented Sept. 10, 1929.

vUNITED STATES PATENT OFFICE.

CARL L. BEAL, F ROCHESTER, NEW YORK, ASSIGNOR T0 EASTMA-N KODAK COMPANY,

OF ROCHESTER, NEW YORK, A CORPORATION OF NEW YORK.

.PROCESS OF' COATING ALUMINUM ELECTROLYTICALLY.

Application led January 17, 1927. Serial No. 161,515.

This invention relates to processes of coating aluminum and alloyscontaining the same, electrolyt-ically. One object of the invention isto provide a rocess in which an electrolytic coating o adequate densityand adherence can be united to an aluminum surface in spite of thetendency of the latter to form a thin resistant film of oxide. Anotherobject of the invention is to provide a process which will accomplishthe above result at the minimum of trouble and expense. Other objectswill hereinafter appear.

In the accompanying drawing,-

Fig. 1 is a diagrammatic vertical section on a reduced scale of one formof apparatus in which the first be conveniently carried out;

Fig. 2 is a similar view of one form of apparatus in which the secondpretreatment step may conveniently be carried out;

Fig. 3 is a similar view of one form of apparatus in which theelectrodeposition may conveniently take place.

It is well known that aluminum has been difficult to coat reliably,because its surface, inzcontact with the air, rapidlyv forms a tenaciousfilm, which even when it is thin, resists the adherence of anelectrodeposited coating over it. For instance, it has been difficult to)roduce a good reliable zinc coating electro ytically on aluminum.

I have found that this trouble can be avoided by first treating' thesurface of. the aluminum in a dilute aqueous alkaline solution,preferably hot, and then making it a cathode in an acid bath, thesesuccessive pretreatment steps bringing the surface t0 a condition inwhich it can bereadily electroplated in a standard commercial bath, suchas an acid-sulfate zinc plating bat i The first pretreatment step doesnot re'- quire any electrolysis, and is very rapid and economical.Various alkaline aqueous so-l lutions maybe employed, such as those ofsodium hydroxid, sodium carbonate, or trisodium phosphate. Theconcentration of the solution may vary considerably. I have found itconvenient to work with one having a hydroxyl ion concentration of thesame order of magnitude as that of a 5% solupretreatment step may tionof trisodium phosphate. Torking with such a solution at a temperatureslightly below boiling, say 190 to 200 F. the surface to be treatedrequires only about a halfl minute immersion. Where it is undesirable tocarry any of this alkaline bath into the bath for the second retreatmentstep, the aricle can be dipped or a few seconds in an acid bath, say 10%aqueous hydrochloric acid, but this is not indispensable, althoughdesirable.

The surface under treatment, after undergoing the action of the alkalinesolution in the first step, is then made a cathode in an aqueous acidsolution. I For example, it may be electrolyzed for about 3 to 5 minutesat a current density of 0.4' amperes per square inch with a. voltage ofaround 2. The acid solution can be, for example, a 5% aqueous sulfuricacid one, but the concentration can obviously be varied considerably,and any other acid can be substituted in an amount which will furnish acorrespondingly sufcient hydrogen ion concentration.

It will be noted that the final cleaning, or second pretreatment stetakes place in an electrolyte which contains substantially noy alkalimetal salts. This is important because any considerable concentration ofthe ionsof such metal causes conditions in the cathode region,.whichfavor further filming of the aluminum 'surface which forms the l biIcathode. In my process the chief cations during the second pretreatmentstep are those of hydrogen (H*) and these'tend to dissolve the oxide andhydroxid that were previously formed or loosened during the first oralkaline pretreatment step. AIoreover, the hydrogen polarization of thecathode surface prevents oxidation. IVhile I do not wish to berestricted to a particular theory of operation. since the abovedescribed pretreatment steps have been found in actual practice to beunexpectedly effective, nevertheless, I believe that hydrogen, inaddition'95 to maintaining a non-oxidizing zone around the cathode, alsomechanically eliminates some of the aluminum hydroxid coating, the

bubbles acting on this coating. after the preliminary attack of thealkaline immersion.

The surface of the aluminum having undergone the pretreatment describedabove, can4 be lated in any well known acid-plating bat i, such as thestandard acid-sulfate 5 zinc plating bath operating at a current densityof 0.1 to 0.2 upon the treated surface.l As such bathsare well known,the lde-4 tails of them are omitted here.

In the drawing, 1 represents any suitable container in which is thedilute aqueous alkaline solution inl which the article 3, having thesurface of the readily oxidizable metal, is suspended in any suitableWay, say by rod 4 and hook. 5. i

1'5 In Fig. 2 any suitable vessel 6 contains the dilute acid solution 7in which the artifcle to be treated 3 is immersed as the cathjode, sayby electroconducting hook 7 from i cathode bar 8. Suitable anodes 9 areconnected to anode bars 10. The bars 8 and .10 are connected to thepoles of any suitable source of current 11.. The anodes 9 are...preferably of a material which does not foul the bath, such ascarbon.

:l In Fig. 3 any suitable vessel 12 has suspended in the bath, as 'thecathode, the treatedarticle 3 suspended by any suitableelectroconducting hook 75 from cathode bar 8. The anodes 13 areconnected to anode bars 14. Bars 8 and 14 are connected to any suitablesource of current 11. In the vesselis ai standard electro-plating bath15 of the type customarily used. In fact, Fig. 3

is merely a diagrammatic representation of currentelectroplatingpractice.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is:

1. In the process of electroplatingl an aluminum surface, the steps oftreating said surface in a dilute aqueous alkaline bath Wit-houtelectrolysis, treating said surface as the cathode in an acid diluteaqueous bath under non-oxidizing conditions, and thereafterelectrodepositing a coating upon said 'treated surface.

day of January 1927.

CARL L. BEAL.

Q. In the .process of electroplating upon an aluminum surface, the stepsof immersing said surface 1n an aqueous alkaline solution to attack thefilm on said surface,

treating said surface as the cathode in a In-theprocess ofelectrodepositing al coating upon an aluminum surface, the steps 6.0 ofimmersng said surface in a hot aqueous f bath having an alkalinity ofthe orderof magnitude of a 5% solution of trisodium phosphate, dippingthe' surface in a dilute Y aqueous acid solution, making said surface ua cathode in an aqueous acid bath, substan-

